A Direct Sequence Code Division Multiple Access (DS/CDMA) cellular communication system, such as the one described in IS-95, is a self interference system. In such a communication system, a number of mobiles and/or portables use the same spectrum in the same geographical area. The signals from the subscriber units are differentiated from each other based on their spreading code (i.e. the user long code PN sequence and the I and Q PN sequences). The capacity limit of such a system is dependent on the amount of self interference in the system. An analogy used to illustrate this point is a cocktail party conversation. If you are at a cocktail party speaking to the person next to you and no one else is in the room with you, you do not have to speak very loud to be heard. When several more people enter the room and start conversing, you have to speak louder to be heard. In other words the self interference has increased and you have to increase your transmitter power to overcome the interference. As more and more people start talking in the room you have to speak louder and louder, and so do the other people in the room, in order to be heard. Eventually, you reach the point where it takes an infinite amount of power to be heard over the other people. That is the capacity limit.
Extending the cocktail analogy, if everyone in the room is hard of hearing you start with a higher level of interference from the other guests than if everyone has normal hearing. Thus, if everyone has better hearing the number of simultaneous conversations that can occur increases, i.e. the system capacity increases. As a result there is a considerable advantage in increasing the receiver's sensitivity in a DS/CDMA system. Any increase in receiver sensitivity directly reduces the amount of transmitter power required and as a result the amount of self interference. Increasing a cellular systems capacity increases an operator's revenue and improves the service the subscriber receives.
The standard receiver in a DS/CDMA system non-coherently detects the transmitted signal. Non-coherent detection does not take into account the phase difference between two transmitted signals. The standard non-coherent receiver first despreads the received signal (i.e. removes the I and Q PN sequences and the user's long code PN sequence) and accumulates a Walsh symbol of data. A Fast Hadamard Transform (FHT) is performed on the despread accumulated data. The FHT essentially correlates the despread signal against the sixty four possible Walsh symbols that could have been sent by the transmitter. The receiver then selects the Walsh symbol with the highest energy (where the energy is determined by summing the square of the I and Q vectors). The non-coherent receiver is an energy detector and does not use the phase of the transmitted signal. It is well known (Sklar, Digital Communications, ISBN 0-13-211939-0, Prentice Hall 1988, p. 161-164) that the bit error rate (BER) performance of coherent demodulation is superior to non-coherent demodulation.
Thus there exists a need for a method and receiver that improves a receiver's sensitivity by approaching a coherent demodulation scheme which exploits the coherence of a channel or an adapted channel over several symbols.